Onion, Crop coefficient, Lysimeter, ET, Irrigation
Onion seedlings (variety: BARI Onion-1) of 35 days old were transplanted on 30 December 2005 in four lysimeter tanks maintaining row to row and plant to plant spacing of 20 cm x 10 cm, respectively. Also, to maintain a favorable atmospheric condition around the lysimetric tanks, the same crop was grown in the fields. The crop was harvested at 31 March 2006. The soil was silty clay loam with field capacity and bulk density, 28% and 1.5 gm/cc, respectively. The selected treatments were;
T1= Irrigation at 10 days interval
T2= Irrigation at 15 days interval
T3= Irrigation at 20days interval
T4= Irrigation at 25 days interval
Since, the micro-lysimeter contains 4 tanks, each having area of 1.0 m x 1.0 m, so no replication of the treatments was possible. Climatic data such as, maximum and minimum temperatures, air humidity, sunshine hours and wind speed together with the location information like elevation, latitude and longitude were incorporated in the software, CROPWAT, to estimate the potential evapo-transpiration.
Measured quantity of water was applied to the tanks to ensure drainage. Prior to irrigation, soil moisture was measured to determine the depleted soil water. During the crop season, no rainfall occurred. Therefore, only the stored soil moisture during the period under consideration were subtracted from the applied water to obtain crop evapo-transpiration (ETc) i.e.,
ETct = Wa + Re– (Dw + Ss)
Where, ETct = crop evapo-transpiration in mm for time period, t
Wa = applied water, mm
Re = effective rainfall, mm, for time t
Dw = drainage water, mm, for time, tSs = stored soil moisture, mm, for time, t
Then from the potential evapo-transpiration (ETo) estimated for the specified period, the value of Kc for the period was determined from the ratio, ETc / ETo.
The crop coefficient values of onion at initial, development, mid season and late season stages were found 0.49, 0.79, 1.03 and 0.8, respectively, from the lysimeter study. As the lysimeter provides a control environment, further study is not needed for confirmation.